Power distribution systems normally can accommodate a variety of plug-in units which plug into various back planes and enable the distribution of various voltage and current levels of power. Given a particular power distribution scheme, the plug-in power units used must function in harmony with one another. This means that they must be properly selected for addition to the backplane. These selections are normally made by a crafts person who is familiar with the requirements of the plug-in units. It is not uncommon for an incorrect unit to be plugged in due to time and other pressures causing plug-in unit preselection to be made incorrectly. This may be due to confusing markings on the plug-in units or failure to understand the proper unit needed for a particular power distribution scheme.